1. Field of the Invention
This invention relates to a system for interconnecting elements of a receiving device of great length, for example a seismic streamer, whereby the number of recording traces corresponding to the sensed signals can be doubled.
2. Prior Art and Technical Considerations
A seismic streamer is usually formed of a plurality of elements of uniform length, serially interconnected. Each of them comprises a great number of seismic sensors (hydrophones) distributed at regular intervals inside a tight sheath.
The sensors of each element are distributed into a certain number of adjacent groups. Each of these groups is formed of an assembly of consecutive sensors distributed over a determined length of the element, whose outputs are electrically interconnected in series or in parallel and which constitutes a seismic receiver.
When the seismic streamer is towed while immersed, the receivers sense the acoustic waves transmitted from a seismic source triggered from a towing vehicle and reflected or refracted by different sub-terranean layers.
The signals generated by each receiver are transmitted to a recording device placed on the vehicle and form a recording "trace". As the seismic receivers are distributed at regular spacings, usually each recorded signal or recording trace is identified by the length l of the streamer element along which are distributed the interconnected sensors of that particular receiver which has generated said recording trace. When interconnecting the sensors distributed over a length of, for example, 6.25 m, inside a streamer element of 75 m, the latter can generate 12 signals or traces corresponding to sensor groups of 6.25 m, or, briefly stated, 12 traces of 6.25 m. When multiplying by two the number of sensors of each group by interconnecting those which are regularly distributed over a length of 12.5 m, the same seismic streamer element is then capable to generate 6 signals or recording traces corresponding to sensor groups of 12.5 m or, briefly stated 6 traces of 12.5 m.
The seismic signals generated by the different receivers may be transmitted in a continuous manner to the recording system by means of particular transmission lines for each of them. But, in view of the present use of longer and longer seismic streamers, the transmission of the different signals generated by the receivers cannot be ensured simultaneously and continuously, but sequentially by making use of a certain number of common transmission lines. In order to effect this sequential transmission, the different seismic signals generated in each element are collected by electronic data acquisition apparatuses placed at regular intervals inside the seismic streamer or inside tight boxes intercalated between streamer elements. The number and the arrangment of the boxes depends on the number of "traces" which can be generated by the seismic receivers of each streamer element and also on the number of channels which can be connected to each acquisition apparatus.
But, usually, the structure of each element is determined by construction and cannot be changed thereafter, the sensors being grouped and the receivers connected to the different inputs of the acquisition apparatuses located at one or the other end of each element. The number of separate seismic signals sensed by each streamer element cannot be easily modified and, accordingly, the number of recording traces corresponding to the signals received by a seismic streamer cannot be increased but by means of additional streamer elements or by manufacturing another type of streamer elements.